(Probably I could have saved this for Friday Weird Science, but I can't help myself, it's getting a lot of press, and hey...who doesn't want two Fridays in a week? Happy Wednesday!)

We all know how it goes.

They stood, framed by stormclouds, together on the windswept cliff. The stranger held Rosamund in an iron grasp, and she thought for a moment he would, indeed, cast her into the waves. But then, as lightning flickered across the sky, she caught his gaze. His eyes lingered on hers, on her tear stained cheeks, on her alabaster neck, pale with cold and damp with rain. In his eyes, she saw a mingling of rage and something else. Was it...desire? Slowly, his hard expression thawed, and Rosamund knew that this night, at least, she was not meant to die.

(Oh yes, some day, I will write one of these...)

Yeah. That's how it goes. The music swells, you look deep into their eyes, and see...what exactly? I don't know about you but I don't ever think I've seen passion in someone's pupils.

We all need ways to measure sexual arousal. No really. Not just for personal reasons. Many studies of sexuality need good, solid, quantifiable measures of sexual arousal, something that correlates well with self-reports.

We do have physical ways of getting this information, of course. They just tend to be ways that drastically decrease your volunteer numbers and increase the difficulty of getting your study past review. For example, to measure penile arousal, you have to actually put a cuff around the penis, and measure distension of the cuff as the erection forms. Vaginal arousal is even more invasive, it often involves sensors on the vaginal all to record increases in moisture. You can imagine how finding out about these measures might shrink your volunteer pool.

Not only that, invasive measures like this might induce bias in the volunteers. It's pretty obvious that you're measuring arousal, and what if you really don't want to show arousal to a particular stimulus? You might start thinking really hard about baseball at some key moments in the study.

So the authors of this study felt that a new measure was needed. One that is less invasive and open to a wide study pool, and one that you can't...well...think away.

So they tried pupil dilation.

They took 320 people (roughly half men and half women) and had them rate themselves according to sexual orientation, ranging between heterosexual, bisexual, or homosexual. They then had the participants look at 30 sec videos of either men or women masturbating while they measured pupil dilation.

And then they made some correlations.

You can see here male and female correlations between pupil dilation in response to the same or to the opposite sex, and measures of the sexual orientation of males and females. In both cases, they got nice correlations, men who rated themselves as toward the straighter end had larger pupil dilation in response to women, and less in response to men. The opposite was true for homosexual men, and bisexual men showed equal pupil responses for both sexes. In women it was a bit more complex, they got pupil dilation for men and women in self-reported straight women. However, this is pretty well established in the literature, that women will respond physically to both sexes. They got further nice correlations going along with less and more arousing sex.

The authors state that the pupil dilation is a reliable measure of sexual orientation, but I think that more strictly it is a measure of sexual arousal in response to specific stimuli, in other words, sexual preference, more than orientation.

And as Deborah Blum pointed out in her coverage of the coverage (meta!) of this story, we don't want to oversell this. After all...pupils dilate in response to a lot of things. For example, if the videos were of higher contrast, dilation would be different. This was something the authors didn't strictly control for. And there are larger confounds. For example, pupil dilation is a signifier of autonomic nervous system response. So...it could be sexual preference...or it could be gas. Pupil dilation is also a good indicator of attention and emotional responses, which are distinct from sexual responses and could present confounds when using this measure in future studies. How do you know the porn you're showing isn't just drawing more attention? Or that the people viewing it are not having strong emotional reactions to it, rather than physical reactions? The desire may be in your eyes, but it might well be mingled with the attention you're paying to that sign, the emotional response to that email, or your desire for a snack.

There's no question that we'll need more studies to make sure that pupil dilation measures are used correctly, and that it really is a reliable measure. The eyes may have it, but time will tell how sensitive they will be.

Possibly due to the resultant mental scaring, I fear I need a bit more explanification of the graph. Is "male sexual orientation" the orientation of males or towards males? What exactly is measured by the horizontal axis?

So the X axis is the 1-6 scale of attraction, starting with 0 (0 attraction to the opposite sex) to 6 (100% attraction to the opposite sex), and the line is the relative correlation in pupil dilation in exposure to same or opposite sex films. At least, as I understand it. The paper is publicly available at PLoS ONE if you want to see the methods for it, there's a lot of data normalization, etc.